Correctly filling a convex and concave mesh from curves
You should retopolgize this mesh. It is pretty simple with method I will present (if this kind a mesh will suite you).

First of all you should make a plane and position it at the bottom of the blade.

Then extrude it all the way up.

Select loop and extrude to the end of axe scaling down each loop.


This gif shows the retopo flow:

If you need more detailed description, let me know in comment what do you need.
Edit: